Electro-Harmonix Memory Boy XO series trim pots settings solution

[VintageGear]

Hi all, a Memory Boy came across my path which was not working. Somebody messed with the internal trim pots. I could not find any information on the net, and many topic starters were puzzled. So I measured a bit myself and found the following to restore the unit to its previous function.


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Adjust trimpots 1 to 4. Voltages should be measured on pin 3 of each BBD. At first I tried half voltage (PSU gives 9.4 volt => 4.7 volt) but that provided distorted sound settings. Around 4.9 volt gave better results in my case. Clearer delay. Ensure you get it right via adjustment of each trimpot, to the decimal point! And check again after adjusting the fourth BBD.

Trimpot 6 should be adjusted with a oscilloscope, it seems to be a mix between both outputs from the BBD chip (not sure!!). Or, sensitive ears. You can hear it go into phase. Adjust it with the DEPTH knob off / lowest setting, and to be sure, the wave form in sine and the expression pedal mode selected.

Trimpot 5 is I think the clocknoise cancellation. Not quite sure, could barely hear any difference when altering this. My oscilloscope is too cheap to note this :-)

Hope this helps anyone....

[PRR]

...Adjust trimpots 1 to 4. Voltages should be measured on pin 3 of each BBD. At first I tried half voltage (PSU gives 9.4 volt => 4.7 volt) but that provided distorted sound settings.....

What is this, a box of BBDs? Then yes, half-supply sounds reasonable but these chips are never quite half-way, and (IME) often a bit to the positive side. I have seen them further off-center than your 4.9 on 9.4.

For a single BBD, apply small signal and twist trimmer. It will cut-out when too low or too high. Mark those points and trim to the middle. Now apply strong signal. It will distort (if not, go stronger). Trim for least distortion, or really (since the flavor changes) least-offensive distortion. That's the sweet-spot.

Doing that on four BBDs could be hairy like balancing the quad carbs on a Honda 750 motorcycle.

[j_flanders]

Half-supply is a good place to start and from there you could tune by ear.

But I find the best and most accurate way is to audio-probe each bbd output. The 'probe points' are clearly labeled on the pcb. There's one for each bbd output:


Apply a sine wave, 400hz for example, to the input of the pedal.
By lack of proper testing equipment I simply record a sine wave from my laptop into a Ditto looper pedal.

Then I use a regular audio probe (piece of shielded wire) for each bbd output test point.
The audio probe, again for lack of better testing equipment, goes to a USB audio interface to a DAW on my computer (= DIY oscilloscope). I use Reaper, running a real time frequency plot plugin. (Span Voxengo).

Now for the interesting part:
-a misbiased bbd gives a distorted output at the probe point
-more distortion = more harmonics
-turn the trimpot until you see the least amount of harmonics in the real time frequency plot: the least amount of spikes at the multiples of your input sine wave...

Repeat for each bbd. Takes 5 minutes.

The clock frequency is cancelled out by combining/mixing the two out of phase bbd signals at the end. There's a probe point for that as well.
When the mix is not exactly 50/50 you'll see a big spike at the clock frequency in the real time frequency plot. Turn the clock trimpot until that peak is at its lowest.

The last trimpot is usually for the repeats/regen. Set it so that oscillation only occurs near the end of the repeats pot.

[StephenGiles]

I've always used Paul's quick and dirty BBD set up method - works really well.